1. Field of the Invention
This invention relates to a vibration wave driven motor, and in particular to a track type resilient member formed into an elliptical shape in which a travelling vibration wave is formed.
2. Related Background Art
Generally, in a vibration wave driven motor utilizing a travelling vibration wave, various shapes of a resilient member formed of a metal material such as stainless steel in which a travelling vibration wave is formed are chosen depending on the purposes of use thereof, and for example, a resilient member formed into an annular shape is known.
A vibration wave driven motor using an annular resilient member of this kind uses a vibration member having a piezo-electric element as an electro-mechanical energy conversion element adhesively secured to the back of the annular resilient member. The piezo-electric element has two piezo-electric element groups differing in polarity, the piezo-electric elements in each group have a positional phase difference of .lambda./2 therebetween, the two groups have a positional phase difference of .lambda./4 therebetween, and an AC electric fields having a time phase difference of 90.degree. therebetween is applied to both piezo-electric element groups, whereby a travelling vibration wave of a wavelength .lambda. is formed in the resilient member by the synthesis of standing waves excited by the piezo-electric element groups.
The travelling vibration wave formed in the resilient member in this manner is utilized to frictionally drive the resilient member and a member which is in pressure contact with the resilient member, whereby the two members can be moved relative to each other.
FIG. 12 of the accompanying drawings is a pictorial perspective view of a linear motor using a track type resilient member formed into an elliptical shape comprising straight portions and arcuate portions. The reference numeral 1 designates a track type resilient member having a piezo-electric element 2 joined to the back thereof. The resilient member 1 is provided on a bottom plate 10 with a vibration insulating member 7 formed, for example, of felt being interposed therebetween. Guide bars 13 are fixed to the widthwise opposite sides of the bottom plate 10. The reference numeral 4 denotes a carriage which is a rectilinearly movable member. Bearings 12 through which the guide bars 13 extend are fixed to the opposite end portions of the carriage 4 so that the carriage 4 is reciprocally rectilinearly movable along the lengthwise direction of the guide bars 13 indicated by arrow A. A slider 11 is provided on the back of the carriage 4 in opposed relationship with one straight portion of the resilient member 1, and this slider 11 is pressed against the one straight portion by a leaf spring 3. AC electric fields having a time phase difference of 90.degree. therebetween as described above are applied from a power source circuit, not shown, to two driving piezo-electric element groups forming the piezo-electric element 2, whereby a travelling vibration wave is formed in the resilient member 1, and the slider 11 brought into pressure contact with the resilient member 1 by the leaf spring 3 is subjected to a frictional force and a thrust is provided in the direction opposite to the direction in which the travelling vibration wave travels, so as to rectilinearly move the carriage 4. By changing over the phase difference between the AC voltages applied to the two driving piezo-electric element groups, the direction in which the travelling vibration wave travels can be changed over and thus, the carriage can be reciprocally moved in the directions indicated by arrow A.
The reason why the track type resilient member 1 is used in such a linear motor is that the area of contact thereof with the slider 11 can be made large and the rectilinear movement distance of the carriage 4 can be made long.
In such a vibration wave driven motor or apparatus, to form a travelling wave in the resilient member 1, it is required that the resonance frequencies of standing waves of the same order excited in the two driving piezo-electric element groups having a positional phase difference of .lambda./4 therebetween be substantially equal to each other.
FIGS. 13 and 14 of the accompany drawings show the states of vibration of the standing waves excited in the resilient member 1, and show the deformed state of the resilient member in a direction perpendicular to the plane thereof by contour lines. Solid lines are the lines of deviation zero (0), i.e., lines (node lines) which are the nodes of vibration. The amount of deviation has its maximum normalized as "1". The numbers of the lines correspond to the altitudes of the lines, and the line number 11 (FIG. 13) or 9 (FIG. 14) is the maximum deviation in the position direction (crest) and the line number 1 is the maximum deviation in the negative direction (valley).
FIGS. 15 and 16 of the accompanying drawings likewise show other standing wave modes, and correspondingly to FIGS. 15 and 16, the perspective views of the states of deviation thereof are shown in FIGS. 17 and 18 of the accompanying drawings, wherein broken lines indicate the shape of the resilient member in its non-vibrated state.
In the vibration member shown in FIGS. 17 and 18, the amplitude formed in the straight portions of the resilient member is divisionally formed on the inner side and the outer side of the resilient member and therefore, as regards the electrode patterns of the piezo-electric elements adhesively secured to the resilient member, with the torsion of the resilient member taken into account, the boundary between the electrodes is provided at the node position of each standing wave, as shown in FIG. 19 of the accompanying drawings, so that vibration may be excited efficiently.
In FIG. 19, the reference characters 2a.sub.1 and 2a.sub.2 designate one driving electrode group for exciting the standing wave mode of FIG. 15, the reference characters 2b.sub.1 and 2b.sub.2 denote the other driving electrode group for exciting the standing wave mode of FIG. 16, the reference character 2G designates and earthing electrode, and the reference characters 2Sa and 2Sb denote electrodes for a sensor for detecting the vibrated states of the standing wave modes excited by said two driving electrode groups. Each of these electrodes is polarization-processed as shown, and the direction of polarization thereof is the direction of the thickness of the piezo-electric element.
Now, in the linear motor as described above, to move the slider 11 smoothly, it is necessary that a travelling wave be formed finely (without amplitude irregularity) in the straight portions of the resilient member 1, and for that purpose, it is necessary that the torsion component during vibration in the straight portions of the resilient member 1 be little, or even if there is more or less torsion component, the direction of the torsion be always the same direction (for example, the vibration of the outer side be always great).
However, in the track type resilient member, it is difficult for the resonance frequencies of two standing waves to coincide with each other and the resilient member is formed by straight portions and arcuate portions. Therefore, as shown in FIGS. 13 to 18, even in the straight portions, a crest and a valley are divisionally formed on the inner side and the outer side of the resilient member. Thus, torsion is created in the straight portions of the resilient member and also the torsion component becomes great. This has led to the problem that the irregularity of the travelling wave becomes great.
Also, when a travelling wave is excited with the modes of FIGS. 13 and 14 superposed one upon the other, the deviation of the outer side of the right-hand straight portion differs in amplitude from location to location. Therefore, this provides irregularity of the travelling wave, thus resulting in a reduction in the feeding speed of the motor and a reduction in the efficiency of the motor as well as the generation of noise.
Further, as shown in FIG. 19, the electrode pattern is a complicated pattern in which the boundary is provided in accordance with the standing wave mode and therefore, great polarization strain remains in the boundary portion and this may lead to the possibility of the piezo-electric elements being destroyed during the polarizing work or during the driving of the motor, and if use is made of an electrode pattern which will avoid this, the electrode pattern will not agree with the standing wave mode and the efficiency of the motor will be reduced.